Catalysts in catalytic converters have been used to decrease the pollution caused by exhaust from various sources, such as automobiles, utility plants, processing and manufacturing plants, airplanes, trains, all terrain vehicles, boats, mining equipment, and other engine-equipped machines. A common catalyst used in this way is the three-way catalyst (“TWC”). The TWC works by converting carbon monoxide, hydrocarbons, and nitrogen oxides into less harmful compounds or pollutants. Specifically, a TWC works by simultaneously reducing the nitrogen oxides to nitrogen and oxygen, oxidizing carbon monoxide to less harmful carbon dioxide, and oxidizing unburnt hydrocarbons to carbon dioxide and water. The prior art TWC is made using at least some platinum group metals. Platinum group metals are defined in this specification to mean platinum, palladium, ruthenium, iridium, osmium, and rhodium in this application unless otherwise stated.
With the ever stricter standards for acceptable emissions, the demand on platinum group metals continues to increase due to their efficiency in removing pollutants from exhaust. However, this demand along with other demands for platinum group metals places a strain on the supply of platinum group metals, which in turn drives up the cost of platinum group metals and therefore catalysts and catalytic converters. Therefore, there is a need for a catalyst that does not require platinum group metals, and has a similar or better efficiency as the prior art catalysts.
Additionally, engines associated with TWC using platinum group metals operate at or near stoichiometric conditions. However, catalysts of the present invention show surprisingly significant improvement in nitrogen oxide reduction performance under rich operating conditions. Further, the present invention improves the reduction of carbon monoxide and hydrocarbon in exhaust by introducing air in between a first catalyst system and a second catalyst system.